C# ArgumentOutOfRangeException for MapID

Hello.
I have 2 class
BlogPost
and
BlogPostDraft

also every class is collections with same name.

On init mongodb set map id to custon property.
But get exception ArgumentOutOfRangeException

its bug or what ?

ps. I dot want store in one collection different type or cast or polymorphism or…

type A → collection A
type B → collection B